The present invention relates in general to control of switches which must not be simultaneously conductive and particularly to the transmit/receive switches for ranging systems.
Laser Infrared Detection and Ranging (LIDAR) systems measure the time between transmission of a pulse of light and the reflection thereof by a target to determine the distance between the light transmitter/receiver and the target. Some LIDAR systems utilize common optical apertures wherein optical elements along the same axis are shared by the transmitter and receiver. Unfortunately, significant quantities of transmitter energy are coupled or reflected into the receiver system by the pulse being transmitted as a result of "backscatter" from these optical components. This backscatter can cause overload and saturation of the receiver during which the system is blind to near-range targets. To minimize this effect, a Transmit/Receive (T/R) switch is used to isolate the receiver from the common optical path during the transmitted pulse duration. The detection of near-range targets requires accurate synchronization of the T/R switch operation with the transmitted pulse. The receiver must be isolated from the common path only long enough to prevent saturation by the backscatter and then reconnected quickly to assure detection of the returning pulse from close range targets. Such return pulses become closer in time to the transmitted pulse as the target range decreases.
In some prior art systems, T/R switch operation is timed by inserting a fixed delay between the transmitter trigger event and the switch control signal. This receiver blanking interval generally equals the sum of the electrical delay inherent in the transmitter firing circuitry, the transmit duration, and the propagation time during which the reflected optical backscatter might have a deleterious effect on the receiver. In practice, the electrical delay in the transmitter firing circuitry between trigger initiation and the transmitter output pulse can change significantly due to temperature shifts and component variations. A blanking interval that is too short allows overload of the receiver resulting in saturation and long recovery times. Alternatively, a blanking interval that is too long undesirably reduces- close range target detection. Hence, the blanking interval is usually set for the longest or worst case delay encountered between trigger initiation and actual transmitter firing over temperature extremes. This results in a sacrifice of near range target detection since the blanking interval and the resulting receiver inactive period will usually be longer than required by the actual firing circuit delay during normal operation at more usual temperatures, for instance.
Another prior art approach requires duplication of the transmitter circuitry requiring compensation. The duplication circuitry is installed in a parallel receiver path requiring synchronization. Assuming similar temperature characteristics in the two paths, the variations in the transmitter firing circuitry are nullified. Also, circuitry with a suitable temperature coefficient can be combined with an adjustable delay generator to track and compensate for temperature induced variations and propagation delay. These prior art approaches while viable in some applications are too complex and costly for other applications.
As previously explained, it is desirable for the T/R switch to be nonconductive during the undesirable backscatter pulse so that the receiver is isolated from the backscatter pulse. This prevents electrical saturation and/or overload of the receiver. An undesirable characteristic of some prior art T/R switches is that they tend to conduct transients to the receiver when switching or when rendered nonconductive. These transients can be induced by the T/R switching control signals. This problem is particularly acute in high power systems using sensitive receivers. The transients can render the system blind to targets until the receiver has recovered therefrom. The undesirable transient feedthrough is a result of the inherent electrical capacitance resistance and/or inductance present in the switching semiconductors and package comprising the T/R switch. Prior art transient suppression is usually accomplished by using a passive network or an active device to absorb or limit the transient energy impulse. These techniques can degrade the signal of interest if the transient has its energy distribution within the time domain response of the system being protected.